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Stern-Fired Torpedoes: An Engineering and Tactical Approach

Capstone Essay Contest

The Seawolf (SSN-21) represents a revolution in submarine technology. This technological revolution precedes what will be a wealth of tactical discussions about its effective employment once the Seawolf is commissioned and put to sea. The advanced capabilities of the Seawolf will change submarine tactics today just as the nuclear-power revolution changed submarine tactics in the 1950s. No one can accurately predict how these tactics will change, but in light of the new technology available, a few expectations can arise, e.g., the ability to fight at higher speeds. Current estimates put the Seawolf at being able to operate acoustically at speeds much higher than submarines have ever been able to attain before. This leads to advantages, but also to additional concerns.

Speed always has been important to submarines, ever since World War II. 1 For a submarine, the lack of speed led to the widespread use of torpedoes that could bridge the gap between the submarine and target quickly. During the development of nuclear-powered submarines, the Soviet Union developed a submarine that was capable of attaining speeds of more than 42 knots. This submarine never entered active service, however, because it was too noisy to be effective in a tactical environment. 2 Acoustic speed became an important factor in the tactical operations of a submarine. Acoustic speed is the maximum speed a submarine can operate at that will not seriously degrade its detection and stealth qualities. Even in the 1980s, submarines could attain acoustic speeds only of around 10 knots.

In an effort to increase the speed of attack, the Russians turned to an old alternative, faster torpedoes. Their rocket torpedo is believed to travel at speeds of about 200-300 knots. This would give one of their attack boats the capability to attack a carrier battle group with ease. However, it remains to be seen whether or not this torpedo will actually work in a combat situation. 3

The United States has focused on increasing the ability of submarines to operate at higher speeds. The Seawolf will be able to attain an acoustic speed of more than 20 knots and possibly 25 knots. This is a revolutionary increase in a submarine's ability to operate, detect, and track an enemy target. 4 But this increase in operational speed also raises engineering concerns about firing of torpedoes at 25 knots. Current tactics call for torpedo launch at 5-10 knots, but at 25 knots, the forces required to launch a torpedo go up exponentially.

If one examines the relation of speed to static pressure, the increase in speed of a submarine results in a square of the pressure increase. For example, at 5 knots, there is .479 pounds-per-square-inch (psi) static pressure on the torpedo. For a 30-inch torpedo tube, 338.6 pounds of force are required to launch a torpedo. Comparatively, at 25 knots, the resultant static pressure on a torpedo tube is about 12 psi—an increase by a factor of 25. The force required to launch a torpedo would be 8,482.3 pounds. With the 3,450 pounds of force required to move the Mk-48 torpedo, a launch at 25 knots requires about 10,000 lbs of thrust. An F/A-18 has about 16,000 lbs of thrust available to it, by comparison.

Since World War II, the Navy has scrapped the notion of stern torpedo tubes in favor of a forward-only arrangement. The Nautilus (SSN-571), the first nuclear submarine, and all constructions since, have only forward tubes. 5 The reason stems from a shift in target acquisition techniques. In World War II-era diesel-electric boats, the periscope was the primary sensor for target tracking and attack. Its 360 degrees arc enabled aggressive boats to attack when closing and when attempting to open range after the initial attack. 6 After the advent of nuclear power, the primary sensor for tracking and engaging an enemy was sonar, with the periscope used for collaboration in surface engagements. Sonar, however, has the unfortunate limitation of being restricted to the forward arc of the boat. The baffles of a submarine, the stern 60° arc of a submarine, make sonar nearly useless, minimizing the value of stern torpedo tubes.

Now, in examining possible changes to submarine construction, one must reevaluate the design and location of submarine torpedo tubes. In a conventional launching system, the air required to blow a torpedo out of the tube at 25 knots will be acoustically unacceptable. In the case of the Seawolf , if the swim-out tubes are used, then the torpedo will expend fuel in order to separate itself from the submarine. This serves to limit the range and effectiveness of the torpedo, because it must speed away from the submarine before becoming active.

This raises another issue of tactical importance, counter-fire. The problem of counter-fire against a submarine after it has launched its weapon is one that is currently unsolved. Snapshots, a quick torpedo launch against a surprise attack, can result in both attacker and defender being sunk. In the case of U.S. submarines, that result is unacceptable. A stern-launched torpedo could be ejected by natural water flow entering the tube and launching it could be made to act quietly and efficiently. This system would take water flowing around the hull and vent it into the torpedo tube. Rather then forcing the torpedo to fight static pressure in exiting the torpedo, the vent system would use water static pressure to eject the torpedo out of the tube. The energy requirements and acoustic noise would be minimal. Once the torpedo is ejected from the tube, start-up would not have to be immediate. Instead, the torpedo could be in stasis until the submarine has put distance between the launch datum and current position. The torpedo could then be activated about .5 nautical mile away from the launching submarine. Once the target begins tracking the torpedo, the target will lack accurate information regarding the position of the launching submarine, reducing the effectiveness of counter-fire techniques. Victory would go more frequently to the attacker capable of detecting the enemy target. If our sensor advantage is maintained, then our submarine fleet will gain an additional technological multiplier against enemy forces.

One problem facing the successful use of stern-fired torpedoes is sonar tracking. The baffles problem still exists and even the new Seawolf cannot eliminate baffles. Since the primary focus of stern-fired torpedoes is aft, the tracking difficulties could mask the target, making it difficult to track and attack. Fire control would have to develop a good target solution and then use that solution as the submarine puts itself into attack position. The other possibility in attack is to use a side-aspect approach, which would close on the target at a wide-aspect angle, close to 090° or 270° relative. This approach affords a good sonar picture, when used with the wide aperture array or towed array sonar. In addition, approach speeds can be regulated with greater ease. Approaching a contact at 25 knots means that a contact at a distance of 10,000 yards will be within 5,000 yards in just six minutes. If sonar cannot be extended much further, then approach speed must be reduced. Shortening the tactical battlespace means a slower relative approach speed. Without compromising ship speed, a wide aspect approach can regulate approach speed by moving forward of the target and then releasing the weapon. 7

The stern-torpedo tube allows for new tactical possibilities, as well as an engineering solution to the problem of launching torpedoes at high speeds. The problem of sonar baffles still exists but this problem can be minimized in a tactical environment. While this weapon system does not currently exist, the possibilities of using stern-fired torpedoes requires an analysis. The submarine force may find itself using the stern-fired torpedo in about a decades or two if the current advances in underwater acoustics advances to allow higher engagement speeds. With the Navy undecided about the design of the next generation of submarines, it would be prudent to research the feasibility of the stern-fired torpedo.

EDITOR'S NOTE: The "Capstone Essays" are chosen from the top papers prepared by Naval Academy midshipmen in The Junior Officer Practicum, which prepares them for roles as junior officers in the four major warfare areas.